The present invention relates to a stator-rotor system of an eccentric screw pump for delivering liquid and/or granular media with an adjustable or readjustable stator.
Eccentric screw pumps are pumps for delivering a plurality of media, in particular viscous, highly viscous and abrasive media such as for example sludges, manure, crude oil and greases. The driven helical rotor moves along in rolling contact in the stator. The latter is a housing with a helically coiled inner side. The rotor performs with its figure axis an eccentric rotary motion around the stator axis. The outer screw, i.e. the stator, has for example the form of a double-lead thread, whereas the rotor screw is only single-lead. The rotor is usually made from a highly abrasion-resistant material, such as steel for example. The stator, on the other hand, is made from an elastic material, for example rubber. As a result of the special formation of rotor and stator, sealed hollow spaces are formed between rotor and stator, said hollow spaces moving along axially as the rotor rotates and delivering the medium. The volume of the hollow spaces is constant, so that the delivery medium is not squashed. With a suitable design, not only fluids, but also solids can be delivered with eccentric screw pumps.
For the formation of the delivery spaces and in order to deliver the given medium with as small a backflow as possible, the rotor lies subject to pressure against an internal wall of the stator formed by an elastic material. On account of the motion of the usually metallic rotor inside the stator usually made of rubber or a similar material, there is a certain amount of abrasion or wear on the stator. As a result of the wear, the pressure-induced contact force is reduced, in particular the contact between the stator and the rotor along an uninterrupted helical contact line cannot be maintained, as a result of which the performance of the eccentric screw pump declines. This applies in particular to pumps which have to overcome a high level of suction. For this reason, the stator has to be exchanged and replaced at regular intervals.
In order to ascertain the time for the replacement of the stator, use is made for example of sensors which detect the wear on the stator on the basis of physical parameters.
Alternatively, embodiments are known, wherein the stator can be readjusted to compensate for the wear. For example, the tension in the stator-rotor system can be adapted by changing the stator diameter.
FIG. 1 shows a diagrammatic partial view of a known stator-rotor system 1 for an eccentric screw pump. Such a system 1 comprises a usually metallic, single-lead helical rotor (not represented) and a stator 3 with a double-lead thread. During the operation of the eccentric screw pump, the rotor performs with its figure axis an eccentric rotary motion about stator longitudinal axis X3. Stator 3 comprises an elastomer part 4 and a stator casing 5 as a support element, wherein there is no fixed connection between elastomer part 4 and stator casing 5.
DE 3433269 A1 describes a stator casing with tensioning devices in the form of tensioning bolts, which are distributed over the entire axial length of the stator casing. This brings about a considerable increase in the weight of the stator-rotor system. In addition, all the tensioning devices have to be individually tightened up for the readjustment.
DE 3641855 A1 describes an adjustable stator with an elastomer body, which is vulcanised in a tubular casing split up at the periphery into segments by the longitudinal slots, and at least one tensioning clamp surrounding the tubular casing.
EP 0292594 A1 discloses a stator casing provided with a longitudinal slot for eccentric screw pumps, which comprises a tensioning device for generating pressure solely in its compression region and for the readjustment in the event of wear on the stator. The tensioning is distributed in part over the length of the stator casing by means of suitable reinforcing ribs.
DE 4312123 A1 describes a stator casing with a plurality of longitudinally running slots, which simplify the readjustment. In order that a readjustment can be better carried out in the region of the pressure-side end of the stator, the slots end shortly before the end of the suction-side end of the stator and run out freely only at the pressure-side end.
DE 4403979 A1 discloses a readjustable stator for eccentric screw pumps with continuous longitudinal slots and longitudinal slots which end at a small distance before the suction-side end of the stator. A continuous slot expediently follows a longitudinal slot in each case.
DE 10200393 A1 describes an eccentric screw pump with a partially tensionable stator. Here, an axially non-continuous tensioning gap is provided in the stator casing surrounding the elastomer stator core. The stud remaining on the entry side forms a tensioning lock on this side. The re-tensioning of the stator takes place by means of a tightening device, which is arranged in the region of the stator casing with a tensioning gap.
Furthermore, according to DE 2331173, a device is known wherein a readjustment of the stator takes place by a partial compression of the stator elastomer on specific lines or points of the stator. For this purpose, the stator comprises helical strips in regions having a particularly high degree of wear. As a result of the displacement of the helical strips, the regions of the inner threaded surface of the stator formed, in cross-section, by the rectilinear sections are in particular changed in their position in the radial direction. Even a severely worn stator lining can thus be deformed in such a way that it again assumes its original cross-sectional shape.
A further possibility makes provision such that a liquid is compressed between the wall of a stator casing and the elastomer part, as a result of which the stator diameter is changed. According to a variant described in U.S. Pat. No. 3,139,035, fluid is introduced into inflatable tubes, as a result of which the pressure on the rotor is increased.
The described prior art has a number of drawbacks. The handling in each given instance is made difficult by the many adjustment options on the various systems. In the described systems, there is in particular no feedback concerning the level of tension between stator and rotor. The adjustment should therefore only be carried out by experienced operatives, because otherwise the risk of incorrect operation is high. If the tension is increased too high, the pump works less well and the wear on the stator is further increased.
With the previously described systems, only compensation of the stator wear is possible, but not an adaptation to the operating conditions prevailing in the given instance.
The problem of the invention is to design the tensioning force of the elastomer of the stator with respect to the rotor in a variable manner in the stator-rotor system of the eccentric screw pump, in order to compensate for the wear on the stator, and wherein the backflow can be kept small even after a long period in operation. Furthermore, influences of the medium on the elastomer in the system should be able to be compensated for.
The above problem is solved by a stator-rotor system and a method for readjusting a stator in a stator-rotor system, which comprise the features in claims 1 and 12. Further advantageous developments are described in the sub-claims.